1. Field of the Invention
The present invention relates in general to the field of electronic reproduction technology and involves an optoelectronic scanning head for drum or flat bed scanning apparatus for point-by-point and line-by-line scanning of opaque and transparency originals.
2. Description of the Prior Art
In drum scanning apparatus, the original which is to be reproduced is mounted on a scanning drum which serves as the original carrier and is read point-by-point and line-by-line by an opto-electronic scanning head which moves axially along the rotating scanning drum. The scanning head essentially contains a scanning objective lens, a diaphragm and an opto-electronic transducer. The original is illuminated point-by-point and the scanning light emanating from the illumination spot on the original is modulated with the information content of the original and passes through the scanning objective and through the diaphragm to the opto-electronic transducer which converts the scanned light into an electrical image signal. When scanning transparency originals, the illumination system is mounted in the inside of the scanning drum which is composed of transparent material and the scanning light is allowed to pass through the transparency original and proceeds into the scanning head and the illumination system and scanning head are synchronously moved in the axially direction of the scanning drum. When scanning opaque originals, the illumination system is a component part of the scanning head and the scan light is reflected by the opaque original and is evaluated by the scanning head. In a flatbed scanning apparatus, the original carrier is a flat planar surface.
So as to scan the originals with optimum image sharpness, the range of focus of the scanning objective must lie in the image planes of the original or, respectively, the distance between the image planes and the scanning objective must always be constant. However, originals which have different thicknesses must be scanned and the scanning drums have different diameter tolerances when manufactured so that the image sharpness must be repeatedly monitored and corrected when necessary. In conventional scanning heads, the correction of the image sharpness is possible only to a limited degree since the scanning objectives can only be moved in a narrow range along their optical axis and this however is not sufficient to achieve an optimum adjustment of the image sharpness and, thus, good scanning quality for all thicknesses of originals do not occur. In order to eliminate this disadvantage, various scanning drums have been proposed which have differences of radiuses which correspond to the most frequently occurring thicknesses of originals. Although the respective distance between the image plane and the scanning objective could thereby be kept nearly constant, this measure is extremely complicated and requires the changing of the scanning drum for originals of different thicknesses.
In the assembly of traditional scanning heads for opaque scanning, the illumination system which is integrated in the scanning head is adjusted and fixed one time in a fashion such that the illumination spot for a rated position of the scanning objective is sharply imaged. However, the sharp imaging of the illumination spot is lost when the scanning objective is moved from its standard position for setting the image sharpness. A further disadvantage of traditional scanning heads is that a corresponding readjustment of the illumination system is frequently not possible at all and can only be implemented with difficulty and with a great expenditure of time by an inexperienced operator.